Many people are confused about how many sets and repetitions you are supposed to do. Most people do 3 sets x 10 repetitions. This is okay if you have the weight and rest in between the repetitions correct. The load and repetition assignments are based on your specific training goals. For Muscular Endurance (toning up muscle) you would want lighter weight. This would be 67% of Max and your repetitions would be 12. For Hypertrophy (Building muscle) you would want a little heavier weight. This would be 67-85% of Max and your repetitions would be 6-12. For Strength you would want heavy weight. This would be 85% of Max and your repetitions would be 6. For Power: single effort event, you would want very heavy weight 80-90% of Max and 1-2 reps. For Power: multiple effort event- you would want heavy weight 75-85% of Max and 3-5 reps. Your sets can very from 2-4 on average. Depending on want kind of work out you are doing it make be more sets or less. But 2-4 sets is most common.
It is important that you realize that strength & power is different than building muscle. Your rest in between repetitions is just as important. For Muscular endurance (toning) you want to only rest less than or equal to 30 seconds. For hypertrophy (building mass) you only want to rest 30 seconds to a minute. This is because you want to break down your muscle and take it to fatigue in order for it to rebuild later. This is how your muscle grows. It breaks down and then rebuilds bigger. For power and strength you rest 2-5 minutes. This should be a full recovery. You will definitely need this longer rest for lower body.
This is important to know when designing a workout program. That way it ensures you reach your goals in the shortest amount of time possible. Also a common mistake is working out the same muscle groups every day or every other day; especially if you are trying to build muscle. By not allowing the muscle to recover you are not allowing it to rebuild and grow. Also for power it doesn’t allow enough rest for the next workload so you are hindering how much weight you can increase weekly or monthly. For muscle endurance it is on the fence whether it helps or hinders. Over a long time frame it will hinder your workout so I would make sure you rest at least a day or two. Another resolution to the problem would be to change up the actually exercises if the muscle groups are the same. Also you will have load increases as you get stronger. This is typically 2.5-5lbs at a time. It can be larger increases but this is usually for larger, stronger athletes. They can increase by 10-15 pound increments.
Also knowing how many times a week to train is important too. A beginner should do resistance training 2-3 times a week. An Intermediate person should do resistance training 3-4 times a week. An advanced person should work out 4-7 times a week. This is basic information and obviously once you get into sport specific needs a lot of the information will vary.